Kanasansoft Web Lab.

var cx=Canvas.getContext(...);
var img=new Image();
img.src="..";
cx.drawImage(img, ...);
cx.toDataURL();

// ==UserScript==
// @name        getFileByDataScheme
// @namespace   http://www.kanasansoft.com/
// @include     *
// ==/UserScript==

(
    function(){

        var parseHTTPHeader=function(responseHeader){
            var headers=responseHeader.split("\n");
            var len=headers.length;
            var parsing=[];
            for(var i=0;i<len;i++){
                if(/^$/.test(headers[i])){
                }else if(/^[\x09\x20]/.test(headers[i])){
                    if(parsing.length==0){
                        throw "SyntaxError:HTTPHeader (first line) "+headers[i];
                    }
                    parsing[parsing.length-1]+="\n"+headers[i];
                }else{
                    parsing.push(headers[i]);
                }
            }
            var len=parsing.length;
            var parsed={};
            for(var i=0;i<len;i++){
                var pair=parsing[i].split(": ",2);
                if(pair.length!=2){
                    throw "SyntaxError:HTTPHeader (format) "+parsing[i];
                }
                if(pair[0] in parsed){
                    throw "SyntaxError:HTTPHeader (repetition) "+pair[0];
                }
                parsed[pair[0]]=pair[1];
            }
            return parsed;
        }

        var loadDataMethod=function(res){
            var headers=parseHTTPHeader(res.responseHeaders);
            if(res.status!=200){
                throw "RequestError"+
                    ":status="+res.status+
                    ":statusText="+res.statusText+
                    ":responseHeaders="+res.responseHeaders;
            }
            var contentType=("Content-Type" in headers)?
                headers["Content-Type"]:
                "application/octet−stream";
            var dataScheme="data:"+contentType+
                ";base64,"+btoa(
                    res.responseText.replace(
                        /[\u0100-\uffff]/g,
                        function(c){
                            return String.fromCharCode(c.charCodeAt(0)&0xff);
                        }
                    )
                );
            data.callback(dataScheme);
        }

        var getData=function(url){
            data.url=url;
            GM_xmlhttpRequest(
                {
                    "method":"GET",
                    "url":data.url,
                    "onload":loadDataMethod,
                    "onerror":loadDataMethod,
                    "overrideMimeType":"text/plain; charset=x-user-defined"
                }
            );
        }

        var data={};

        var getFileByDataScheme=function(url,callback){
            data.callback=callback;
            getData(url);
        }

        if(window.self==window.top){
            var url=prompt("image url","");
            if(url==null||url==""){
                return;
            }
            var handler=function(url,callback){
                return function(){
                    getFileByDataScheme(url,callback);
                }
            }
            var callback=function(dataScheme){
                var elem=document.createElement("img");
                elem.src=dataScheme;
                document.body.appendChild(elem);
            }
            window.addEventListener(
                "load",
                handler(url,callback),
                false
            );
        }

    }
)();